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<br />EM 1110-2-1913 <br />31 Mar 78 <br /> <br />berm can afford protection against underseepage pressures with less <br />length and thickness than the other types of seepage berms, its cost is <br />generally much greater than the other types, and thus it is rarely <br />specified. <br /> <br />c. Berm Design. Design equations, criteria, and examples are <br />presented in Appendix C for seepage berms. <br /> <br />5-5. Pervious Toe Trench. <br /> <br />a. General. Where a levee is situated on deposits of pervious <br />material overlain by little or no impervious material, a partially pene- <br />trating toe trench, as shown in figure 5-2, can improve seepage <br /> <br />WATERSIDE: <br /> <br />It <br />I <br />, <br /> <br />LANDS IDE <br /> <br /> <br /> <br />THIN IMPf:RVIOU$ STRATUM <br /> <br />. PERVIOUS STRATUM <br /> <br />Figure 5-2. Typical partially penetrating <br />pervious toe trench <br /> <br />conditions at or near the levee toe. Where the pervious stratum is <br />thick, a drainage trench of any practicable depth would attract only a <br />small portion of the seepage flow and detrimental underseepage would <br />bypass the trench. Consequently, the main use of a pervious toe trench <br />is to control shallow underseepage and protect the area in the vicinity <br />of the levee toe. Pervious toe trenches are often used in conjunction <br />with relief well systems; the wells collect the deeper seepage and the <br />trench collects the shallow seepage. Such a system is shown in fig- <br />ure 5-3. The trench is frequently provided with a perforated pipe to <br />collect the seepage. The use of a collector system is dependent on the <br />volume of seepage and, to some degree, the general location of the levee. <br /> <br />5-4 <br /> <br />e <br /> <br />. <br /> <br />i <br /> <br />e <br /> <br />. <br /> <br />.. <br /> <br />e <br />